Muscle
This attribute represents the character's raw strength and power.
This affects how much damage is inflicted in combat, and generally helps with tasks of brute force, such as pulling one's self up from a ledge, holding a door shut against intruders, lifting a 10 ton boulder above one's head, or crushing your opponents' skulls like little tin cans.
A strong character need not have giant, rippling muscles. One could have above-average strength thanks to genetic experimentation, cybernetic implants, or supernatural powers. A little girl with a Muscle of 8 is a perfectly acceptable and interesting character concept.
A character with a poor Muscle score is a wimp, barely able to carry a backpack, and always need others to open the pickle jar. A character with a good Muscle score is a powerhouse: capable of untold feats of strength including busting down doors, carrying comrades, and tossing enemies.
Some examples of muscular people include weightlifters, furniture movers, and Hercules.